graphics/gstreamer1-plugins-gl: revert incomplete libglvnd/X11=off

This reverts commit 1cf7a05cba19b1c400e2c87bd0b0696ed92d4b0a.

$ pkg-config gstreamer-gl-1.0 --variable=gl_winsys
egl wayland x11

$ poudriere testport -j 131amd64 graphics/gstreamer1-plugins-gl
[...]
0x0000000000000001 NEEDED Shared library: [libX11-xcb.so.1]
0x0000000000000001 NEEDED Shared library: [libX11.so.6]
[...]